1. Field of the Invention
This invention relates to techniques for reducing acoustic feedback, and more particularly relates to such techniques in which a digital notch filter algorithm is employed.
2. Description of Related Art
Digital notch filters have been used in the past in an attempt to reduce acoustic feedback in sound amplification systems, including public address systems. For example, U.S. Pat. No. 4,091,236 (Chen, issued May 23, 1978) describes an analog notch filter for an audio signal to suppress acoustical feedback. The apparatus receives an audio signal which is substantially non-periodic in the absence of acoustical feedback and substantially periodic with an instantaneous dominant frequency in the presence of such feedback. The duration of successive periods are monitored and compared by an up/down counter to determine whether the audio input signal is substantially periodic and to determine the instantaneous dominant frequency of the audio signal. Upon detection of an audio signal which is substantially periodic, the notch filter is tuned to the instantaneous dominant frequency so as to suppress the acoustical feedback.
U.S. Pat. No. 4,232,192 (Beex, issued Nov. 4, 1980) describes an integrator/detector (FIG. 9) which determines when an audio signal has exceeded a threshold for a selected number of cycles. If the threshold is exceeded for the selected number of cycles, a sampler circuit samples a voltage corresponding to the frequency that has exceeded the threshold. The sampled voltage is used by a voltage frequency converter in order to adjust the notch of a notch filter implemented in hardware.
U.S. Pat. No. 5,245,665 (Lewis et al., issued Sep. 14, 1993) describes a device for suppressing feedback in which a Fast Fourier Transform is conducted on samples of digitized signals to produce corresponding frequency spectrums. The magnitudes of the spectrum at various frequencies are analyzed to determine one or more peak frequencies which are 33 decibels greater than harmonics or sub-harmonics of the frequency in an attempt to detect resonating feedback frequencies. Two processors are required. A primary processor periodically collects a series of the passing digital signals and conducts a Fast Fourier Transform on each collected series of digital signals. The frequency spectrums produced by the Fast Fourier Transform are examined by the primary processor to discover the presence of any resonating feedback frequency. Filter control signals are passed by the primary processor, along with the digital sound signals, to a secondary processor which operates a digital filtering algorithm in accordance with the filter control signals to attenuate resonating feedback frequencies in the stream of digital signals.